Spatio-temporal Variation Of Sediment Extracellular Enzyme Activity And Its Effect On Greenhouse Gas Production In Yangtze Estuary Wetlands

Wetland ecosystem has the dual functions of"sink"and"source".It is a key component of the global material cycle and has a significant impact on regulating the global climate.As a biochemical reaction catalyst,extracellular enzymes play a key role in regulating the material cycle rate.The Yangtze estuary wetland is the largest estuarine wetland in China.Many scholars have studied the mechanism of carbon and nitrogen cycle and the influencing factors of greenhouse gas emissions in this region.However,the distribution characteristics of extracellular enzyme activities in sediments and their effects on the material cycle in this region are still unclear.Therefore,studying the spatio-temporal variation of sediment extracellular enzyme activities and their effects on greenhouse gas production in Yangtze estuary wetlands will help to improve the understanding of carbon and nitrogen cycle mechanism in the Yangtze estuary wetland ecosystem,and more accurately to manage the regional greenhouse gas production and emission.In this study,the typical intertidal wetland of the Yangtze Estuary was selected as the research area.?-glucosidase(BG),polyphenol oxidase(PPO),urease(UE)and N-acetylglucosidase(NAG)were selected as the research objects.On the basis of one-year field observation and laboratory analysis,we discussed the spatio-temporal variation of sediment extracellular enzyme activity,the environmental factors of sediment extracellular enzyme activity,and the effect of sediment extracellular enzyme on greenhouse gas production in Yangtze Estuary wetlands.The main research conclusions are as follows:(1)The activities of BG,NAG,UE and PPO in the Yangtze River Estuary Wetland varied from 0.45 mg·g^-1·day^-1 to 1.80 mg·g^-1·day^-1,1.66 mg·g^-1·day^-1to 3.69 mg·g^-1·day^-1,0 mg·g^-1·day^-1to 0.14 mg·g^-1·day^-1,and 6.38 mg·g^-1·day^-1 to 16.30 mg·g^-1·day^-1,respectively.Different types of sediment extracellular enzymes have different temporal and spatial characteristics.In terms of seasonal changes,the activities of the four extracellular enzymes all showed the order of autumn>summer>winter,and the enzyme activities in winter were significantly lower than other seasons(P<0.05).In terms of vertical changes,only PPO had a rule that the activity of the surface layer was significantly higher than that of the middle and bottom layers(P<0.05).In terms of spatial point changes,the activities of BG and PPO showed significant changes(P<0.05),the activity of BG showed as follows:Wusongkou>Xitan>Dongtan>Nanhui vegetated mudflat>Nanhui mudflat>Qidong,performance of PPO activity was Nanhui mudflat>Qidong>Dongtan>Nanhui vegetated mudflat>Xitan>Wusongkou.Vegetation affected the distribution of sediment extracellular enzyme activities,but no significant difference of sediment extracellular enzyme activities was found between vegetated and non vegetated areas(P>0.05).(2)In the Yangtze estuary wetland,the activity of sediment extracellular enzyme was affected by the physical and chemical properties of sediments,the physical and chemical properties of overlying water,and the interaction of extracellular enzymes.For the physical and chemical properties of sediments,redundancy analysis showed that p H,organic carbon(TOC),extracted sulfate(SSO₄^2-),electrical conductivity and microbial biomass carbon(MBC)were the main influencing factors of extracellular enzyme activities.For the physical and chemical properties of overlying water,salinity and sulfate(SO₄^2-)concentration have significant positive correlations with the activity of PPO(P<0.01).In addition,there was a mutual regulation relationship between extracellular enzymes.The activities of BG,NAG and UE were significantly positively correlated(P<0.01),while the activities of PPO and BG were weakly negatively correlated(P<0.05).(3)The variation range of nitrous oxide(N₂O)emission fluxes of sediments under non-flooded and flooded conditions were-12.36?160.34 nmol·m^-2·h^-1,-16.12?192.12nmol·m^-2·h^-1,respectively.The N₂O emission flux under two conditions has the same seasonal variation characteristics:higher in summer and winter,lower in autumn.The variation range of methane(CH₄)emission flux under non-flooded and flooded conditions were 0.01?2.20?mol·m^-2·h^-1,-0.03?2.32?mol·m^-2·h^-1,respectively.The seasonal and spatial changes of CH₄ flux were different under two states.The carbon dioxide(CO₂)emission flux of sediment under flooded conditions varies from 0?mol·m^-2·h^-1 to 399.14?mol·m^-2·h^-1,which is lower than the emission flux under non-flooded conditions(0?mol·m^-2·h^-1 to 1474.96?mol·m^-2·h^-1),both of them reached the highest emission level in summer,and their spatial changes were different.There was a significant positive correlation between BG activity and CH₄ emission flux under non-flooding condition(P<0.01),while N₂O and CO₂ emission process were more complex,and the effect of extracellular enzyme activity on N₂O and CO₂ emission flux was not obvious(P>0.05).(4)In the anaerobic slurry culture experiment,the total amount of N₂O in the culture bottle increased first and then decreased to zero;the total amount of CH₄ and CO₂ increased at the early stage of culture,but gradually leveled off at the later stage;the activity of BG decreased in the surface and middle layer,but increased first and then decreased in the bottom layer;the changes of NAG activity and UE activity were similar,both of them increased first and then decreased;the changes of PPO activity were more disorderly,but the overall trend of activity was downward.NAG affects the production of N₂O,and its activity has a very significant positive correlation with the amount of N₂O in the culture bottle(P<0.01).It was also found that salinity affected the production of greenhouse gases and the activity of extracellular enzymes in sediments.The production of N₂O was less and the production of CH₄ was significantly higher at low salinity(0.5‰),and the activity of NAG was higher at medium salinity(15‰).In addition,the vertical depth affects the amount of greenhouse gas production in the sediments.As the nutrients decreased with sediment depth,the production of greenhouse gas in the upper layer was higher than that in the lower layer.